A typical proximity switch consists of a sensor, a signal-processing unit, and an output port. The signal-processing unit detects the distance of the sensor from the surface of a target object, the “activating surface”. In prior art designs, when this distance is less than a certain magnitude, the “activation distance”, the output port is switched to its “on” state.
Previous proximity switches have had either a fixed activation distance or have required manual calibration to set the activation distance. In actuality, the signal received from the sensor has an intensity that may be affected by distance, but that can also be affected by color and other environmental circumstances. An exemplary traditional use for such proximity switches is as a user interface input mechanism for physically challenged persons with limited motoric capability. A user of a previous, analog proximity switch may typically position the sensor near the eyebrow with emitted light aimed below the eyebrow when the face is in a relaxed position. Thenceforth, the user can simply lower the eyebrow into the emitted light to raise the intensity of the signal due to darker color (assuming a dark eyebrow) and greater proximity. Alternatively, a user can aim the emitted light between folds of skin beside the eye that contract during a winking movement. In this case, the intensity of reflected light raises primarily due to greater proximity.
The aforementioned techniques avoid non-purposive triggering of the switching mechanism due to some involuntary movements, such as blinking, but can be subject to non-purposive triggering due to other involuntary movements, such as tremors that can follow a purposive movement. Also, some users may have less difficulty in operating the switch merely by closing the eye, rather than winking or lowering the eyebrow. However, the target area for aiming the sensor is diminished by inability of the sensor to distinguish between mere blinks and purposive closing of the eyelid alone. Thus, the need remains for a signal processing unit that can distinguish between purposive and non-purposive signals. The present invention fulfills this need.